Insight into the role of reduced graphene oxide in enhancing photocatalytic hydrogen evolution in disordered carbon nitride

Abstract

Compared to crystalline carbon nitride, the performance of disordered carbon nitride (d-CN) as a hydrogen production photocatalyst is extremely poor. Owing to its disordered atomic orientation, it is prone to numerous defect states. These energy states are potential sites for trapping and recombination of photogenerated charge carriers. As a result, rapid recombination of photogenerated charge carriers places a fundamental photophysical challenge in charge separation and transport, which inhibits its photocatalytic activity. In the presence of reduced graphene oxide (rGO), d-CN shows enhanced photocatalytic production of hydrogen. However, photophysical insight into the tacit role of rGO is not well understood which limits the rational design of d-CN as a photocatalyst. Particularly, understanding of the early time-scale (in fs to ps) recombination mechanism and the charge transport kinetics has not yet been achieved. With the help of ultrafast transient absorption spectroscopy, femtosecond time-resolved photoluminescence spectroscopy and transient photocurrent measurements, this article deciphers the ultrafast dynamics of the separation and transport of photogenerated charge carriers in d-CN facilitated by rGO. It is found that rGO substantially suppresses the bimolecular and trap-assisted recombination and enables a faster separation of charge carriers. As a result, it increases the lifetime of the charge carriers to be transported to the surface catalytic sites, and therefore, augments the rate of hydrogen production almost by an order of magnitude. Our findings therefore offer a proof-of-concept for overcoming the trap-mediated recombination problems in disordered carbon nitride.